The present invention relates to a transcoder device using a compression encoding format for a motion picture and, more particularly, to a transcoder device for transcoding a compressed and encoded bitstream of the motion picture in a syntax level. Furthermore, the present invention relates to a motion picture communication system including the transcoder device.
In recent years, for example, International Telecommunication Union Telecommunication Standardization sector (ITU-TS) recommendations H.261 and H.263, and Moving Picture Experts Group (MPEG)-4 internationally standardized by International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) are known as compression encoding formats for efficiently transmitting motion picture signals within narrower bandwidths.
The compression encoding formats for motion pictures, such as H.261, H.263, and MPEG-4, adopt hybrid compression in which reduction in the redundancy in a time-base direction by the use of motion-compensated interframe prediction is combined with reduction in the redundancy in a spatial direction by the use of discrete cosine transform (DCT) and variable length coding (VLC) using Huffman codes.
However, different bitstreams are generated in the compression and encoding in different compression encoding formats for motion pictures.
Accordingly, when a motion-picture encoding device uses a compression encoding format different from that in a motion-picture decoding device, it is necessary to provide a transcoder device that transcodes an encoded bitstream transmitted from the motion-picture encoding device into an encoded bitstream that can be correctly decoded by the motion-picture decoding device, in order to realize video communication between the motion-picture encoding device and the motion-picture decoding device.
FIG. 1 is a block diagram showing a typical structure of a transcoder device 7 in which a decoding unit 71 is tandem-connected to an encoding unit 72. The decoding unit 71 decodes an encoded bitstream transmitted from a motion-picture encoding device. The encoding unit 72 outputs an encoded bitstream that can be correctly decoded by a motion-picture decoding device. The encoded bitstream that is received from the motion-picture encoding device is input in the decoding unit 71 that decodes the received encoded bitstream into a motion picture signal. The decoded motion picture signal is supplied to the encoding unit 72 that is required to generate an encoded bitstream again.
The decoding unit 71 includes a reception buffer 701, a variable-length decoder 702, an inverse quantizer 703, an inverse discrete cosine transformer 704, an adder 707, a frame memory 705, and a motion compensation predictor 706.
The reception buffer 701 temporarily stores motion-picture encoded data 718 that is received. The variable-length decoder 702 decodes the motion-picture encoded data. The inverse quantizer 703 inversely quantizes the decoded data. The inverse discrete cosine transformer 704 performs inverse discrete cosine transform (inverse DCT) for the output from the inverse quantizer 703. The frame memory 705 stores a motion picture signal 719. The motion compensation predictor 706 detects a motion vector on the basis of the decoded data and the motion picture signal 719 and performs motion compensation prediction. The decoding unit 71 outputs the decoded motion picture signal 719.
The encoding unit 72 includes a subtractor 708, a discrete cosine transformer 709, a quantizer 710, a variable length encoder 711, a transmission buffer 712, an inverse quantizer 713, an inverse discrete cosine transformer 714, an adder 715, a frame memory 716, and a motion compensation predictor 717.
The subtractor 708 subtracts an output from the motion compensation predictor 717 from the received motion picture signal 719. The discrete cosine transformer 709 performs the DCT for the output from the subtractor 708. The quantizer 710 quantizes the output from the discrete cosine transformer 709. The variable length encoder 711 encodes the output from the quantizer 710. The transmission buffer 712 temporarily stores the encoded data supplied from the variable length encoder 711. The inverse quantizer 713 inversely quantizes the output from the quantizer 710. The inverse discrete cosine transformer 714 performs the inverse DCT for the output from the inverse quantizer 713. Since the structure of the decoding unit 71 and the encoding unit 72 shown in FIG. 1 is well known, a detailed description of the components in the decoding unit 71 and the encoding unit 72 is omitted herein.
Even when the motion-picture encoding device uses the same compression encoding format as in the motion-picture decoding device, encoding tools including a resynchronization marker (RM), data partitioning (DP), and reversible variable length coding (RVLC) are defined in the MPEG-4in order to prevent the quality of the motion picture after the decoding from largely degrading if, for example, a bit error occurs in the encoded bitstream.
The use of the encoding tools is not essential and it is possible for a user to arbitrarily determine whether the above encoding tools are used. However, different syntaxes are defined for the encoded bitstreams in the MPEG-4 depending on the used encoding tools.
As described above, even when the motion-picture encoding device uses the same compression encoding format as in the motion-picture decoding device, the user can arbitrarily determine whether, for example, the encoding tools are used. In addition, parameter values used in the encoding can be arbitrarily set. Different syntaxes are used for the encoded bitstream depending on the arbitrary determination or the arbitrary setting.
Hence, even when the motion-picture encoding device uses the same compression encoding format as in the motion-picture decoding device, the transcoder device described above is required to realize the video communication between the motion-picture encoding device and the motion-picture decoding device.
In the transcoder device described above, the decoding unit for decoding an encoded bitstream transmitted from the motion-picture encoding device is connected to the encoding unit for outputting an encoded bitstream that can be correctly decoded by the motion-picture decoding device. Since the received encoded bitstream is input in the decoding unit that decodes the encoded bitstream into a motion picture signal and the decoded motion picture signal is input in the encoding unit that generates an encoded bitstream again, an extremely large amount of calculation is required. The extremely large amount of calculation causes increase in the circuit size and the power consumption.
Furthermore, when the motion-picture encoding device uses the same compression encoding format as in the motion-picture decoding device and the above transcoder device is not provided, there is a problem in that the encoded bitstream output from the motion-picture encoding device cannot be correctly decoded by the motion-picture decoding device depending on whether the encoding tools are used or how the parameter values used in the encoding are set.